Does a spell cast instead of an opportunity attack still observe the timing of an opportunity attack?

The War Caster feat states that

When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, instead of making an opportunity attack.

The rules of the opportunity attack state that

The attack occurs right before the creature leaves your reach.

Should the former override the latter? That is, does the ruling given as to order of resolution in the rules of opportunity attacks still hold even when the opportunity attack never actually takes place?

Xanathar’s Guide to Everything states that

If you’re unsure when a reaction occurs in relation to its trigger, here’s the rule: the reaction happens after its trigger completes, unless the description of the reaction explicitly says otherwise.

So, can I perform the action described in At what point in a creature's movement does an opportunity attack take place?, given this information?

Attack of opportunity

We had a recent question in my party today regarding when you can make an attack of opportunity. One group believes that you only threaten squares after you have come up in the initiative order for that turn, and thus don’t threaten squares and can’t make attacks of opportunity until after your turn. The other group believes you can make an attack of opportunity when anyone leaves your threatened square (one per round unless otherwise stated) before or after your turn. Who is right?

Can one use the Bigby’s Hand spell as part of the War Caster reaction opportunity attack?

My friend and I are having a debate about this. The war caster feat says:

The spell must have a casting time of 1 action and must target only that creature.

I believe this is to eliminate AoE spells. Can I use my reaction to cast Bigby’s Hand with the War Caster feat?

My friend’s argument is that the spell itself summons the hand, which you can then use one of its effects, and these effects don’t necessarily target a creature (example, interposing hand just provides half cover). My argument is that, since as part of the casting you can choose an effect and certain effects only target one creature, that you’d be allowed to use one of those effects only.

Also, if it’s already cast, using one of Bigby’s hands’ effects is only a bonus action, whereas the War Caster feat says that you can cast a spell with the casting time of one action. Technically, the spell does have the casting time of one action, but it’s already been cast, and the attack would only be a bonus action. Would one of the effects of Bigby’s Hand be allowed on the War Caster’s opportunity attack if it were already cast?

Can you really stack all of this on an Opportunity Attack?

I’ve stumbled upon a character idea which has quite the potentially to screw someone if they run from you, and I want to know if it is legal in the rules.

A high elf rogue/cleric with the booming blade cantrip and war caster. In theory you get the damage from the weapon, sneak attack (if not used this round), divine strike (if you have it) and booming blade once you hit and the moment he moves…

As example, a rogue 3/cleric 8 character, with rapier would get 7d8 (1 rapier, 2 booming blade, 1 divine strike and 3 once he moves) + 2d6 (sneak attack) + dex bonus.

Is everything correct here per the rules?

If so, I think that’s pretty insane for something that doesn’t really costs any resources.

5E war casting feat attack of opportunity

When an enemy disengages one of your allies in combat, does this provoke an attack of opportunity to all PC’s that can see the enemy and respond (i.e. weapon ready) including with long-range attacks like from a range with a readied bow or is it that only the player that the enemy was in melee with at the time of disengaging get the attack of opportunity?

scenario – monk in melee combat, a ranger about 50ft away with a readied longbow, and a sorcerer about 45ft with the war caster feat and an attack spell with a range of 60ft. Would the sorcerer be able to cast a long-range reaction attack spell and ranger get an attack of opportunity in responce if the enemy disengages the monk?

Opportunity attack and class features during own turn. (via Dissonant Whispers)

Consider this scenario: a college of swords bard uses dissonant whispers on a creature within 5 feet of him while he is holding a melee weapon. The creature fails the save and has to immediately use its reaction to move away. In my understanding, this movement provokes opportunity attacks. So:

  1. Would the caster also be entitled to an opportunity attack?

  2. If so, would he be able to augment his opportunity attack with a blade flourish?